Improve Example03Markers with navigation and info window features

[afarber]

This PR overhauls Example03Markers to showcase more library features and adds a new OnInfoWindowCloseListener to the library.

Library changes

• Add OnInfoWindowCloseListener - fires when info window is closed (manually or via auto-dismiss)
• Add Marker.showInfoWindow() and Marker.hideInfoWindow() for programmatic control
• Add UiSettings.infoWindowAutoDismiss to auto-close info windows after a duration
• Bump version to 0.12.0

Example03Markers rewrite

• Replace add/remove/clear buttons with prev/next marker navigation
• Add FAB to toggle info window on selected marker
• Add status toolbar showing selection index, marker title, and camera state
• Status text turns red when info window is open (demonstrates close listener)
• Clicking a marker selects it and animates camera to center
• Extract reusable components: MarkerToolbar, StatusToolbar, Constants, MarkerData
• Use real Bochum landmark locations as POIs